The SCID-hu core will be a resource to all investigators in this vaccine program to test in vivo: 1. The effectiveness of different neutralizing antibodies generated by vaccines in the prevention of HIV-1 infection by primary isolates from different clades. 2. Determine the long term effects of passively administered vaccine generated antibodies on the appearance of gp120 mutations in the vaccine corresponding epitope areas that occur in vivo. Such studies will yield invaluable information on the biological broadness of vaccine generated antibodies and on the development of virus resistance. SCID-hu mice will be constructed in the core and made available to all vaccine investigators. The core technician will assist investigators in technical aspects of experiments such as introduction of infection and collection of study specimen.